Pressurized compressible fluids, such as atmospheric air, carbon dioxide, helium, argon, nitrogen, liquids, etc., are commonly used to deliver energy in the form of pressure in a variety of industrial applications. The devices that use the pressurized fluid, known as load devices, include robots, paint applicators, turbines, power generators, jet engines, pneumatic tools, and others. Compressible fluids are typically pressurized using a compressor, which may take one of many forms, such as a centrifugal compressor, a reciprocating compressor, a rotary screw, a stack of alternating rotors and stators, or other forms.
A compressor takes in a compressible fluid at an inlet, uses energy to compress a mass of the compressible fluid to a smaller volume and higher pressure, then discharges the fluid thus compressed through an outlet. An individual compressor produces compressed fluid at a specified flow capacity, defined in terms of volume of free fluid at the inlet of the compressor per amount of time. The individual compressor also produces a selected discharge pressure at the outlet due to the normal operation of the compressor. The selected discharge pressure can typically be varied up to a specified maximum discharge pressure of which the compressor is capable.
The specified flow capacity and selected discharge pressure are chosen to suit the particular application for which the compressor is intended. For example, some typical compressors intended for an automobile manufacturing and assembly plant have selected discharge pressures in the general range of 95 to 125 pounds per square inch gage (PSIG), and a flow capacity in the range of 1,000 to 3,000 standard cubic feet per minute (SCFM). SCFM is defined as, “cubic feet of volume per minute at the standard conditions of 14.7 pounds per square inch absolute (psiA) and 60 degrees Fahrenheit.” Many other ranges of discharge pressures and flow capacity are possible depending on the needs of the particular application.
Each load device in turn has a demand flow rate, which is the volume rate of fluid used by the load device in its operation. Each load device also has a specified incoming pressure that it requires for normal operation. Demand flow rate may be fairly constant or change frequently, depending on the application. Any load device is likely to drop its demand flow rate temporarily at least occasionally for interruptions such as maintenance, breaks, etc.
For facilities in which many load devices are operating, it is common to provide the required pressurized fluid to the load devices through a single fluid distribution system which services the load devices at its downstream outlets. The single distribution system can in turn be serviced by any number of compressors that supply pressurized fluid to the distribution system at the system's upstream inlets. This single distribution system provides greater flexibility than if each load device had to be serviced by its own compressor, acting to average-out any changes in demand flow rate.
However, total demand flow rate of a collection of load devices still tends to fluctuate during operation. The degree of fluctuation depends on the type and operational nature of the facility using the load devices. If too few compressors are operated, when the demand flow rate rises particularly high, it will surpass the flow rate from the compressors. This will lower the distribution pressure, disrupting the proper operation of the load devices.
To prevent disruptions of this sort, multiple compressor systems are generally designed and installed to cater to the maximum peak demand flow rate at the required load pressure. Facility operators tend to operate the maximum installed capacity of all compressors all the time at the maximum pressure, to ensure that the load devices receive enough pressure even during peaks in demand flow rate. So, the installed compressor discharge flow capacity is greater than it usually needs to be; and the compressors must be set to a higher discharge pressure than what the load devices require most of the time. Excessive compressor capacity and discharge pressure both translate into higher energy consumption, maintenance costs, and capital costs.
However, successful operation of the load devices is typically a greater priority than efficient operation of the compressors. The traditional multi-compressor system therefore sacrifices compressor system efficiency to prevent pressure shortages during times of peak demand flow rate.
A multiple compressor system is therefore desired in which the flow rate from the compressors is varied to match variations in demand flow rate, preferably without operating compressors at partial capacity. It is also desired to provide a multiple compressor system with improved efficiency, in which energy consumption, maintenance costs, and capital costs are reduced without reducing capacity to deliver sufficiently pressurized fluid to the load devices.